سعال اختي راح شوي بعد ما لبست كمامة بالباص.

This is the Arabic idafa structure, which is the usual way to show possession.

• سعال = cough
• اختي = my sister

Put together, سعال اختي literally means the cough of my sister, which in natural English is my sister’s cough.

Arabic does not use an apostrophe like English does.

Because in an idafa phrase, the first noun normally does not take الـ. Its definiteness comes from the second part.

So:

• سعال اختي = my sister’s cough
• not السعال اختي

Even though the whole phrase is definite, the first noun stays without الـ.

Because the grammatical subject is سعال = cough, not اختي = my sister.

The verb agrees with سعال, and سعال is masculine, so you get راح.

If you used كحة instead, which is feminine, you would normally say راحت:

• كحة أختي راحت شوي

Basically yes, but in Levantine it is often used with symptoms, pain, problems, and similar things to mean:

• went away
• subsided
• got better

So سعال اختي راح شوي means the cough improved a bit or went away a little, not that it physically traveled somewhere.

شوي means a little, a bit, or somewhat. Here it softens the statement:

• راح شوي = went away a bit
• not completely, just partially

It is one of the most common everyday words in Levantine.

بعد ما means after when it introduces a clause with a verb.

So:

• بعد ما لبست كمامة = after she wore a mask

This is a very common Levantine pattern. Learners often first know بعد as after/later, but بعد ما is especially common before a full verb phrase.

Arabic often leaves out subject pronouns because the verb usually gives enough information, and context fills in the rest.

So instead of saying:

• بعد ما هي لبست كمامة

the sentence simply says:

• بعد ما لبست كمامة

Adding هي is possible for emphasis, but it is not necessary in a neutral sentence.

Yes, that is a good observation. In normal Arabic writing, short vowels are not written, so a form like لبست can look ambiguous on the page. Context usually tells you the intended meaning.

Here, because the sentence is about my sister’s cough, the natural reading is after she wore a mask.

Because كمامة is indefinite here, meaning a mask.

• لبست كمامة = she wore a mask
• لبست الكمامة = she wore the mask

So the sentence is talking about wearing some mask, not a specific previously mentioned one.

بالباص is بـ + الباص.

In Levantine, بـ is commonly used for location and for being in or on transportation. So بالباص means on the bus or in the bus.

You may also hear في الباص, which is possible too, but بالباص is very natural in colloquial speech.

Possibly, yes. A more colloquial version might be:

كحة أختي راحت شوي بعد ما لبست كمامة بالباص

Or:

خفّت كحة أختي شوي بعد ما لبست كمامة بالباص

These sound more fully conversational because كحة is more everyday than سعال. The original sentence is still understandable, but it has a slightly more formal noun inside an otherwise colloquial sentence.